I. Field of the Invention
This invention relates generally to apparatus for effecting intervertebral body arthrodesis, and more particularly to an improved cage construction for reducing movement of adjacent vertebral bodies to thereby enhance bone growth and fusion.
II. Discussion of the Prior Art
Intervertebral body arthrodesis or fusion of a spine segment is indicated for symptomatic patients with intervertebral disk disease. The purpose of the fusion in these cases is to eliminate or reduce the amount of motion at that site and, possibly, the source of pain. For solid bone fusion between vertebral bodies, it is imperative that interbody movement be prevented if bone growth and fusion is to result.
When a vertebral body or disk is damaged, the spinal cord or nerve roots may be impinged. This causes extreme pain to the person, either in the back, neck or extremities. When all conservative drug treatments and minimally invasive procedures have been exhausted. A spinal stabilization with fusion may be the answer. It has long been thought that interbody fusion achieves relief of the patient's pain by correction of an existing mechanical deformation to its anatomic baseline and by the provision of stability to the spine segment to prevent future abnormal, i.e., excessive or pain-provoking motions. Spinal interbody fusion has, in theory, been the preferred surgical technique to achieve these goals. Unfortunately, this procedure has demonstrated widely varying results, with fusion rates ranging from 19 percent to 95 percent and satisfactory clinical results ranging from 14 percent to 93 percent. Devices, referred to as "fusion cages" have been developed to help stabilize adjacent vertebral bodies to be fused and to help promote solid bone fusion between such vertebral bodies. Fusion cages have been designed to correct existing mechanical deformation, to provide stability to the vertebral bodies until arthrodesis is achieved, provide the best possible environment for successful arthrodesis and to achieve this with limited morbidity associated with their use.
Ideally, interbody cage devices should restore disk height, place the annular fibers in a "normal" tension, create lordosis through the joined vertebral bodies, obtain sagittal balance through the segment, reduce subluxed facet joints, enlarge the neuorforaminal space, and restore to normal the proportion of weight bearing through the anterial spinal column.
Presently, there are two types of cages, round and square. Such cages vary in size for the area of the spine in which they are to be implanted, i.e., lumbar, thoracic or cervical. FIG. 1 illustrates a typical prior art fusion cage, such as the so-called BAK cage available from SpineTech of Minneapolis, Minn., and the RAY threaded fusion cage from Surgical Dynamics of Norwalk, Conn. These cages are threaded titanium cylinders having a longitudinal bore extending through them and with holes through the walls of the cylinder located at the roots of the threads. The longitudinal bore can be packed with bone particles and the holes allow bony ingrowth through the cages during the post-operative healing phase.
Depending upon the location of the damaged disk, either one or two such cages may be implanted between adjacent vertebral bodies. Generally, in the lumbar region, two such cage devices are employed and they are positioned on either side of the mid-line sufficiently far apart that they do not touch one another and generally will be about 3 mm from the anterior and posterior cortical margin of the end plates comprising the vertebral body. The cylindrical cages come in several diameters and the diameter chosen again depends upon the anatomy of the spine at the point of placement.
In implanting such devices, semi-circular laminotomies are performed on both lateral sides of the disk space and then the bone of the plateaus of the adjacent vertebral bodies are drilled and tapped to receive the threaded cylinders therein. Before closure, autologous bone graft material, usually harvested from the patient's iliac crest is packed within the hollow confines of the cylindrical threaded cage devices to enhance the opportunity for solid bone fusion.
It is imperative for successful fusion that stability of the adjacent vertebral bodies be maintained as bone growth takes place, even when the patient has returned to his/her daily activities. Motion through the operative segments leads to progressive mechanical loosening and eventual failure of implant or bone. Motion significantly decreases the chances of obtaining a solid bony arthrodesis. While the RAY threaded fusion cage and the BAK fusion cage constitute an improvement over earlier intercorporeal bone graft in spinal fusion following disk removal, instances have been reported where the cylindrical threaded cages move out of position, especially if the patient's daily activities involve bending from side-to-side or front-to-rear. When the body leans forward, the disk between the vertebral bodies compress on the anterior side of the spinal column while the posterior disk stretches. When the body leans to the left, the left side of the disk compresses and the right side stretches. Just the opposite occurs when leaning to the right. When two cylindrical threaded cages are placed in between the vertebral bodies following removal of the disk therebetween, the compression and stretching of the disk is gone, but the vertebral bodies open slightly as explained above. With the radius of the cylindrical threaded cages, rotation can exist from side-to-side. When vertebral body can roll over the side of the cylindrical cage, but this will not happen on a forward or backward movement, because there is no radius curve over the length of the cage member. A need exists for a fusion cage design that prevents front-to-back and side-to-side shifting of the adjacent vertebral bodies as the patient bends or leans. An overview of cage devices presently in use for interbody fusion are set forth in an article entitled "Spine Update--Lumbar Interbody Cages", by Bradley K. Weiner, M.D., Spine, Vol. 23, No. 5, Mar. 1, 1998, pp. 634-640.